Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
827 AM EDT Sun Aug 05 2018

Valid 00Z Mon Aug 06 2018 - 00Z Mon Aug 13 2018

A broad inverted mid/upper-level trough continues to push west of
the islands toward the International Dateline. At this point, all
eyes are on Hurricane Hector, currently a Category 3, generally on
a westward path along 15N latitude. As of the latest advisory from
the National Hurricane Center, the cone of uncertainty mid-week
skirts the southern fringe of the Big Island. Relative to the
official track, the 00Z CMC is decidedly weaker and slower while
the 00Z UKMET is a hair quick. In spite of being a compact
tropical cyclone, any northward adjustment to its path would bring
further impacts to the Big Island. As Hector exits the longitude
of Hawai`i by late Thursday, the pattern flattens out a bit before
the next potential tropical development (currently around 119W)
may near the region just beyond next weekend. However, forecast
spread is tremendous and beyond the period of interest.

At the surface, an anticyclone centered north-northwest of Hawai`i
will continue to encourage a steady period of east-northeasterly
trades to the region, generally in the 15 to 20 knot range. Such
winds will gradually pick up across the southern most islands,
particularly over the Big Island with the 06Z GFS showing
northeasterly winds reaching 30 to 40 knots by early Wednesday.
The northern extent of the tropical circulation will contain the
strongest wind field although the cyclone remains on the smaller
side. By late Thursday onward, climatological trades re-assume
their dominance across the region, although slightly weaker than
earlier in the week. The 06Z GFS appears to be on the lower end of
the spectrum given a northeastward displaced dome of high pressure
across the east-central Pacific.

Precipitable water values are below climatology to start the week
with most locations in the 1 to 1.25 inch range. Would expect
coverage of trade driven showers to be slightly reduced in
response. Thereafter, the atmosphere moistens up considerably in
response to the passage of Hurricane Hector. While a bulk of the
heavy rainfall will stay south of Hawai`i, models do show enhanced
chances for precipitation across the Big Island on Wednesday. In
its wake, some solutions show a secondary spike in moisture
leading to additional rainfall chances over the Big Island late
Thursday into Friday. Into next weekend, drier air spills in as
trade wind strength remains modest at best. Consequently, expect
diminished shower coverage.


Rubin-Oster